You can tap an untapped creature you haven’t controlled continuously since the beginning of your most recent turn to convoke a spell.

10/5/2018

Convoke doesn’t change a spell’s mana cost or converted mana cost.

10/5/2018

When calculating a spell’s total cost, include any alternative costs, additional costs, or anything else that increases or reduces the cost to cast the spell. Convoke applies after the total cost is calculated.

10/5/2018

Because convoke isn’t an alternative cost, it can be used in conjunction with alternative costs.

10/5/2018

Tapping a multicolored creature using convoke will pay for or one mana of your choice of any of that creature’s colors.

10/5/2018

When using convoke to cast a spell with in its mana cost, first choose the value for X. That choice, plus any cost increases or decreases, will determine the spell’s total cost. Then you can tap creatures you control to help pay that cost. For example, if you cast Worldsoul Colossus (a spell with convoke and mana cost ) and choose X to be 3, the total cost is . If you tap two green creatures and two white creatures, you’ll have to pay .

10/5/2018

If a creature you control has a mana ability with in the cost, activating that ability while casting a spell with convoke will result in the creature being tapped before you pay the spell’s costs. You won’t be able to tap it again for convoke. Similarly, if you sacrifice a creature to activate a mana ability while casting a spell with convoke, that creature won’t be on the battlefield when you pay the spell’s costs, so you won’t be able to tap it for convoke.